Internal combustion engines are well-known and widely used for providing power for vehicle propulsion, power generation, and still other applications where it is desirable to rotate parts in machinery. A great many different strategies for fueling internal combustion engines, ranging from different fuel types to different mechanisms for delivering fuel to engine cylinders, have been proposed over the years. Certain designs mix fuel with air in the intake conduit to an engine housing, with the fuel and air charge commonly being spark ignited within individual cylinders. Other common designs inject fuel directly into an engine cylinder. So-called direct injection fueling strategies are typically used in compression ignition diesel engines. One characteristic of compression ignition diesel engines is the need to increase pressure of the fuel to a relatively high injection pressure prior to delivery into relatively highly compressed air within an engine cylinder.
Decades ago engineers developed a fuel system known as a common rail where a fuel reservoir is maintained at or close to a desired injection pressure. A plurality of individual fuel injectors fluidly connected to the common rail can be supplied with the fuel at rail pressure and selectively operated to effect fuel injection. In more recent years, a variation on the common rail design was developed where a plurality of separate fuel accumulators are positioned fluidly between a common rail and each of a plurality of fuel injectors. The plurality of accumulators are coupled together in a so-called daisy chain arrangement, with the overall apparatus still commonly referred to as a common rail or common rail-type fuel system.
Despite advances in common rail and related fuel system technologies, engine systems are still in widespread use where unit pumps are provided as a part of or coupled with each individual fuel injector. In a typical unit pump or unit injector design each of the fuel injectors in the engine is equipped with a cam-actuated fuel pump that provides pressurized fuel for injection. Variations on the cam-actuated design include the incorporation of various control valves to at least partially decouple a timing and manner of fuel injection from the rotation of the cam. Both common rail systems and unit pump strategies can produce heat from the intense pressurization of the fuel and friction between moving components, in some instances producing some challenges to sufficient cooling of the equipment. U.S. Pat. No. 8,480,009 proposes a low-leakage large-bore fuel system having a common rail fluidly connected to different types of fuel. A plurality of fuel injectors are fluidly connected to the common rail and each includes a cooling inlet and a cooling outlet.